Geophysical exploration methods for metallic minerals, water-bearing strata, hydrocarbon deposits and for engineering applications, may include electrical resistivity surveys. In such a survey, employing a quadripole configuration, a pair of current electrodes are planted in the ground, spaced apart by several tens of meters. A pair of spaced-apart voltage electrodes are established in the ground between the two current electrodes. An electrical current is applied to the two current electrodes for a period of time such as several seconds or several tens of seconds and is then abruptly switched off. During application of the current, a voltage difference will be observed across the voltage electrodes. When the current is cut off, the voltage difference as seen at the voltage electrodes will slowly decay over several seconds. The decay time is a measure of the resistivity of the earth. The above technique, known as the Induced Potential (IP) method may be applied horizontally over the ground surface, or vertically, in a borehole. The simplistic explanation given above is subject to many variations such as different electrode arrangements, electrical measurement techniques and the like, depending upon the type of information sought, degree of sophistication required, and overall cost of the survey.
In order to interpret the results of an IP survey or of any other type of electrical prospecting, it is necessary to study, in the laboratory the petrophysical characteristics of typical rock-type specimens. Of interest are such matters as resistivity per unit volume of a rock in terms of porosity, mineral content, permeability, formation pressure, temperature and fluid content. Changes in the electrical properties of reservoir rock are indicative of either a physical change to the rock structure from overburden stress, resulting from pore pressure depletion, or changes in the composition of saturating fluids. Resistivity measurements can be a useful tool for production as well as exploration.
For laboratory tests, typically, the porous rock specimen is in the form of a cylindrical core an inch or two in diameter and a few inches long. The core is inserted into an insulating core barrel such as plastic, rubber or other insulating material. The two end faces are contacted by the current electrodes, usually via a brine solution that saturates the pore space. Silver/silver chloride (Ag/AgCl) voltage electrodes contact the core between the current electrodes to read the potential difference across a unit length of the core to measure the resistivity. It is preferable to mount the voltage electrodes well inside the current electrodes to avoid polarization effects that may distort the measurements.
One such laboratory device is described in a paper by Vinegar and Waxman in Geophysics V49, n8, August 1984, pp. 1267-1287. A tutorial monograph Experimental and Theoretical Aspects of Induced Polarization by J. Bertin and J. Loeb is published by Geopublication Associates and Gebrueder Barntraeger of Berlin, West Germany.
When studying core samples from boreholes, it is desirable to reproduce the in-situ conditions of reservoir pressure and temperature. In particular, laboratory equipment capable of simulating an overburden equivalent to a 20,000-foot or more depth is not believed to be presently available. Most such equipment provides resistivity measurements at a pressure not much greater than ambient atmospheric pressure. It is the purpose of this invention to provide a core barrel for use in making resistivity measurements of core specimens at great overpressures of at least 10,000 psi (pounds per square inch) with means for injecting selected interstitial fluids representative of various reservoir cconditions in the borehole.